CORE JAVA INTERVIEW QUESTIONS AND ANSWERS

Q: What is the difference between an Interface and an Abstract class?
A: An abstract class can have instance methods that implement a default behavior. An
Interface can only declare constants and instance methods, but cannot implement default
behavior and all methods are implicitly abstract. An interface has all public members and no
implementation. An abstract class is a class which may have the usual flavors of class members
(private, protected, etc.), but has some abstract methods.
Q: What is the purpose of garbage collection in Java, and when is it used?
A: The purpose of garbage collection is to identify and discard objects that are no longer
needed by a program so that their resources can be reclaimed and reused. A Java object is
subject to garbage collection when it becomes unreachable to the program in which it is used.
Q: Describe synchronization in respect to multithreading.
A: With respect to multithreading, synchronization is the capability to control the access of
multiple threads to shared resources. Without synchonization, it is possible for one thread to
modify a shared variable while another thread is in the process of using or updating same shared
variable. This usually leads to significant errors.
Q: Explain different way of using thread?
A: The thread could be implemented by using runnable interface or by inheriting from the
Thread class. The former is more advantageous, 'cause when you are going for multiple
inheritance..the only interface can help.
Q: What are pass by reference and passby value?
A: Pass By Reference means the passing the address itself rather than passing the value.
Passby Value means passing a copy of the value to be passed.
Q: What is HashMap and Map?
A: Map is Interface and Hashmap is class that implements that.
Q: Difference between HashMap and HashTable?
A: The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized
and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt
allow). HashMap does not guarantee that the order of the map will remain constant over time.
HashMap is non synchronized and Hashtable is synchronized.
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Q: Difference between Vector and ArrayList?
A: Vector is synchronized whereas arraylist is not.
Q: Difference between Swing and Awt?
A: AWT are heavy-weight componenets. Swings are light-weight components. Hence swing
works faster than AWT.
Q: What is the difference between a constructor and a method?
A: A constructor is a member function of a class that is used to create objects of that class.
It has the same name as the class itself, has no return type, and is invoked using the new
operator.
A method is an ordinary member function of a class. It has its own name, a return type (which
may be void), and is invoked using the dot operator.
Q: What is an Iterators?
A: Some of the collection classes provide traversal of their contents via a java.util.Iterator
interface. This interface allows you to walk a collection of objects, operating on each object in
turn. Remember when using Iterators that they contain a snapshot of the collection at the time the
Iterator was obtained; generally it is not advisable to modify the collection itself while traversing
an Iterator.
Q: State the significance of public, private, protected, default modifiers both singly
and in combination and state the effect of package relationships on declared items
qualified by these modifiers.
A: public : Public class is visible in other packages, field is visible everywhere (class must be
public too)
private : Private variables or methods may be used only by an instance of the same class that
declares the variable or method, A private feature may only be accessed by the class that owns
the feature.
protected : Is available to all classes in the same package and also available to all subclasses of
the class that owns the protected feature.This access is provided even to subclasses that reside
in a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public private or protected).It
means that it is visible to all within a particular package.
Q: What is an abstract class?
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A: Abstract class must be extended/subclassed (to be useful). It serves as a template. A
class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class
may contain static data. Any class with an abstract method is automatically abstract itself, and
must be declared as such.
A class may be declared abstract even if it has no abstract methods. This prevents it from being
instantiated.
Q: What is static in java?
A: Static means one per class, not one for each object no matter how many instance of a
class might exist. This means that you can use them without creating an instance of a class.Static
methods are implicitly final, because overriding is done based on the type of the object, and static
methods are attached to a class, not an object. A static method in a superclass can be shadowed
by another static method in a subclass, as long as the original method was not declared final.
However, you can't override a static method with a nonstatic method. In other words, you can't
change a static method into an instance method in a subclass.
Q: What is final?
A: A final class can't be extended ie., final class may not be subclassed. A final method can't
be overridden when its class is inherited. You can't change value of a final variable (is a
constant).
Q: What if the main method is declared as private?
A: The program compiles properly but at runtime it will give "Main method not public."
message.
Q: What if the static modifier is removed from the signature of the main method?
A: Program compiles. But at runtime throws an error "NoSuchMethodError".
Q: What if I write static public void instead of public static void?
A: Program compiles and runs properly.
Q: What if I do not provide the String array as the argument to the method?
A: Program compiles but throws a runtime error "NoSuchMethodError".
Q: What is the first argument of the String array in main method?
A: The String array is empty. It does not have any element. This is unlike C/C++ where the
first element by default is the program name.
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Q: If I do not provide any arguments on the command line, then the String array of
Main method will be empty of null?
A: It is empty. But not null.
Q: How can one prove that the array is not null but empty?
A: Print args.length. It will print 0. That means it is empty. But if it would have been null then
it would have thrown a NullPointerException on attempting to print args.length.
Q: What environment variables do I need to set on my machine in order to be able to
run Java programs?
A: CLASSPATH and PATH are the two variables.
Q: Can an application have multiple classes having main method?
A: Yes it is possible. While starting the application we mention the class name to be run.
The JVM will look for the Main method only in the class whose name you have mentioned. Hence
there is not conflict amongst the multiple classes having main method.
Q: Can I have multiple main methods in the same class?
A: No the program fails to compile. The compiler says that the main method is already
defined in the class.
Q: Do I need to import java.lang package any time? Why ?
A: No. It is by default loaded internally by the JVM.
Q: Can I import same package/class twice? Will the JVM load the package twice at
runtime?
A: One can import the same package or same class multiple times. Neither compiler nor
JVM complains abt it. And the JVM will internally load the class only once no matter how many
times you import the same class.
Q: What are Checked and UnChecked Exception?
A: A checked exception is some subclass of Exception (or Exception itself), excluding class
RuntimeException and its subclasses.
Making an exception checked forces client programmers to deal with the possibility that the
exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method•
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Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its
subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't
force client programmers either to catch the exception or declare it in a throws clause. In fact,
client programmers may not even know that the exception could be thrown. eg,
StringIndexOutOfBoundsException thrown by String's charAt() method• Checked exceptions
must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.
Q: What is Overriding?
A: When a class defines a method using the same name, return type, and arguments as a
method in its superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is
called, and not the method definition from superclass. Methods may be overridden to be more
public, not more private.
Q: What are different types of inner classes?
A: Nested -level classes, Member classes, Local classes, Anonymous classes
Nested -level classes- If you declare a class within a class and specify the static modifier, the
compiler treats the class just like any other -level class.
Any class outside the declaring class accesses the nested class with the declaring class name
acting similarly to a package. eg, outer.inner. -level inner classes implicitly have access only to
static variables.There can also be inner interfaces. All of these are of the nested -level variety.
Member classes - Member inner classes are just like other member methods and member
variables and access to the member class is restricted, just like methods and variables. This
means a public member class acts similarly to a nested -level class. The primary difference
between member classes and nested -level classes is that member classes have access to the
specific instance of the enclosing class.
Local classes - Local classes are like local variables, specific to a block of code. Their visibility is
only within the block of their declaration. In order for the class to be useful beyond the declaration
block, it would need to implement a more publicly available interface.Because local classes are
not members, the modifiers public, protected, private, and static are not usable.
Anonymous classes - Anonymous inner classes extend local inner classes one level further. As
anonymous classes have no name, you cannot provide a constructor.
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Q: Are the imports checked for validity at compile time? e.g. will the code containing
an import such as java.lang.ABCD compile?
A: Yes the imports are checked for the semantic validity at compile time. The code
containing above line of import will not compile. It will throw an error saying,can not resolve
symbol
symbol : class ABCD
location: package io
import java.io.ABCD;
Q: Does importing a package imports the subpackages as well? e.g. Does importing
com.MyTest.* also import com.MyTest.UnitTests.*?
A: No you will have to import the subpackages explicitly. Importing com.MyTest.* will import
classes in the package MyTest only. It will not import any class in any of it's subpackage.
Q: What is the difference between declaring a variable and defining a variable?
A: In declaration we just mention the type of the variable and it's name. We do not initialize
it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are
both definitions.
Q: What is the default value of an object reference declared as an instance variable?
A: null unless we define it explicitly.
Q: Can a level class be private or protected?
A: No. A level class can not be private or protected. It can have either "public" or no
modifier. If it does not have a modifier it is supposed to have a default access.If a level class is
declared as private the compiler will complain that the "modifier private is not allowed here". This
means that a level class can not be private. Same is the case with protected.
Q: What type of parameter passing does Java support?
A: In Java the arguments are always passed by value .
Q: Primitive data types are passed by reference or pass by value?
A: Primitive data types are passed by value.
Q: Objects are passed by value or by reference?
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A: Java only supports pass by value. With objects, the object reference itself is passed by
value and so both the original reference and parameter copy both refer to the same object .
Q: What is serialization?
A: Serialization is a mechanism by which you can save the state of an object by converting it
to a byte stream.
Q: How do I serialize an object to a file?
A: The class whose instances are to be serialized should implement an interface
Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a
fileoutputstream. This will save the object to a file.
Q: Which methods of Serializable interface should I implement?
A: The serializable interface is an empty interface, it does not contain any methods. So we
do not implement any methods.
Q: How can I customize the seralization process? i.e. how can one have a control over
the serialization process?
A: Yes it is possible to have control over serialization process. The class should implement
Externalizable interface. This interface contains two methods namely readExternal and
writeExternal. You should implement these methods and write the logic for customizing the
serialization process.
Q: What is the common usage of serialization?
A: Whenever an object is to be sent over the network, objects need to be serialized.
Moreover if the state of an object is to be saved, objects need to be serilazed.
Q: What is Externalizable interface?
A: Externalizable is an interface which contains two methods readExternal and
writeExternal. These methods give you a control over the serialization mechanism. Thus if your
class implements this interface, you can customize the serialization process by implementing
these methods.
Q: What happens to the object references included in the object?
A: The serialization mechanism generates an object graph for serialization. Thus it
determines whether the included object references are serializable or not. This is a recursive
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process. Thus when an object is serialized, all the included objects are also serialized alongwith
the original obect.
Q: What one should take care of while serializing the object?
A: One should make sure that all the included objects are also serializable. If any of the
objects is not serializable then it throws a NotSerializableException.
Q: What happens to the static fields of a class during serialization? Are these fields
serialized as a part of each serialized object?
A: Yes the static fields do get serialized. If the static field is an object then it must have
implemented Serializable interface. The static fields are serialized as a part of every object. But
the commonness of the static fields across all the instances is maintained even after serialization.
Q: Does Java provide any construct to find out the size of an object?
A: No there is not sizeof operator in Java. So there is not direct way to determine the size of
an object directly in Java.
Q: Does importing a package imports the subpackages as well? e.g. Does importing
com.MyTest.* also import com.MyTest.UnitTests.*?
A: Read the system time just before the method is invoked and immediately after method
returns. Take the time difference, which will give you the time taken by a method for execution.
To put it in code...
long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();
System.out.println ("Time taken for execution is " + (end - start));
Remember that if the time taken for execution is too small, it might show that it is taking zero
milliseconds for execution. Try it on a method which is big enough, in the sense the one which is
doing considerable amout of processing.
Q: What are wrapper classes?
A: Java provides specialized classes corresponding to each of the primitive data types.
These are called wrapper classes. They are e.g. Integer, Character, Double etc.
Q: Why do we need wrapper classes?
A: It is sometimes easier to deal with primitives as objects. Moreover most of the collection
classes store objects and not primitive data types. And also the wrapper classes provide many
utility methods also. Because of these resons we need wrapper classes. And since we create
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instances of these classes we can store them in any of the collection classes and pass them
around as a collection. Also we can pass them around as method parameters where a method
expects an object.
Q: What are checked exceptions?
A: Checked exception are those which the Java compiler forces you to catch. e.g.
IOException are checked Exceptions.
Q: What are runtime exceptions?
A: Runtime exceptions are those exceptions that are thrown at runtime because of either
wrong input data or because of wrong business logic etc. These are not checked by the compiler
at compile time.
Q: What is the difference between error and an exception?
A: An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error.
These JVM errors and you can not repair them at runtime. While exceptions are conditions that
occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file
does not exist. Or a NullPointerException will take place if you try using a null reference. In most
of the cases it is possible to recover from an exception (probably by giving user a feedback for
entering proper values etc.).
Q: How to create custom exceptions?
A: Your class should extend class Exception, or some more specific type thereof.
Q: If I want an object of my class to be thrown as an exception object, what should I
do?
A: The class should extend from Exception class. Or you can extend your class from some
more precise exception type also.
Q: If my class already extends from some other class what should I do if I want an
instance of my class to be thrown as an exception object?
A: One can not do anytihng in this scenarion. Because Java does not allow multiple
inheritance and does not provide any exception interface as well.
Q: What happens to an unhandled exception?
A: One can not do anytihng in this scenarion. Because Java does not allow multiple
inheritance and does not provide any exception interface as well.
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Q: How does an exception permeate through the code?
A: An unhandled exception moves up the method stack in search of a matching When an
exception is thrown from a code which is wrapped in a try block followed by one or more catch
blocks, a search is made for matching catch block. If a matching type is found then that block will
be invoked. If a matching type is not found then the exception moves up the method stack and
reaches the caller method. Same procedure is repeated if the caller method is included in a try
catch block. This process continues until a catch block handling the appropriate type of exception
is found. If it does not find such a block then finally the program terminates.
Q: What are the different ways to handle exceptions?
A: There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions.
and
2. List the desired exceptions in the throws clause of the method and let the caller of the method
hadle those exceptions.
Q: Q: What is the basic difference between the 2 approaches to exception
handling...1> try catch block and 2> specifying the candidate exceptions in the throws
clause?
When should you use which approach?
A: In the first approach as a programmer of the method, you urself are dealing with the
exception. This is fine if you are in a best position to decide should be done in case of an
exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions,
then do not use this approach. In this case use the second approach. In the second approach we
are forcing the caller of the method to catch the exceptions, that the method is likely to throw.
This is often the approach library creators use. They list the exception in the throws clause and
we must catch them. You will find the same approach throughout the java libraries we use.
Q: Is it necessary that each try block must be followed by a catch block?
A: It is not necessary that each try block must be followed by a catch block. It should be
followed by either a catch block OR a finally block. And whatever exceptions are likely to be
thrown should be declared in the throws clause of the method.
Q: If I write return at the end of the try block, will the finally block still execute?
A: Yes even if you write return as the last statement in the try block and no exception
occurs, the finally block will execute. The finally block will execute and then the control return.
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Q: If I write System.exit (0); at the end of the try block, will the finally block still
execute?
A: No in this case the finally block will not execute because when you say System.exit (0);
the control immediately goes out of the program, and thus finally never executes.
Q: How are Observer and Observable used?
A: Objects that subclass the Observable class maintain a list of observers. When an
Observable object is updated it invokes the update() method of each of its observers to notify the
observers that it has changed state. The Observer interface is implemented by objects that
observe Observable objects.
Q: What is synchronization and why is it important?
A: With respect to multithreading, synchronization is the capability to control
the access of multiple threads to shared resources. Without synchronization, it is possible for one
thread to modify a shared object while another thread is in the process of using or updating that
object's value. This often leads to significant errors.
Q: How does Java handle integer overflows and underflows?
A: It uses those low order bytes of the result that can fit into the size of the type allowed by
the operation.
Q: Does garbage collection guarantee that a program will not run out of memory?
A: Garbage collection does not guarantee that a program will not run out of memory. It is
possible for programs to use up memory resources faster than they are garbage collected. It is
also possible for programs to create objects that are not subject to garbage collection
Q: What is the difference between preemptive scheduling and time slicing?
A: Under preemptive scheduling, the highest priority task executes until it enters the waiting
or dead states or a higher priority task comes into existence. Under time slicing, a task executes
for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then
determines which task should execute next, based on priority and other factors.
Q: When a thread is created and started, what is its initial state?
A: A thread is in the ready state after it has been created and started.
Q: What is the purpose of finalization?
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A: The purpose of finalization is to give an unreachable object the opportunity to perform
any cleanup processing before the object is garbage collected.
Q: What is the Locale class?
A: The Locale class is used to tailor program output to the conventions of a particular
geographic, political, or cultural region.
Q: What is the difference between a while statement and a do statement?
A: A while statement checks at the beginning of a loop to see whether the next loop iteration
should occur. A do statement checks at the end of a loop to see whether the next iteration of a
loop should occur. The do statement will always execute the body of a loop at least once.
Q: What is the difference between static and non-static variables?
A: A static variable is associated with the class as a whole rather than with specific
instances of a class. Non-static variables take on unique values with each object instance.
Q: How are this() and super() used with constructors?
A: Othis() is used to invoke a constructor of the same class. super() is used to invoke a
superclass constructor.
Q: What are synchronized methods and synchronized statements?
A: Synchronized methods are methods that are used to control access to an object. A
thread only executes a synchronized method after it has acquired the lock for the method's object
or class. Synchronized statements are similar to synchronized methods. A synchronized
statement can only be executed after a thread has acquired the lock for the object or class
referenced in the synchronized statement.
Q: What is daemon thread and which method is used to create the daemon thread?
A: Daemon thread is a low priority thread which runs intermittently in the back ground doing
the garbage collection operation for the java runtime system. setDaemon method is used to
create a daemon thread.
Q: Can applets communicate with each other?
A: At this point in time applets may communicate with other applets running in the same
virtual machine. If the applets are of the same class, they can communicate via shared static
variables. If the applets are of different classes, then each will need a reference to the same class
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with static variables. In any case the basic idea is to pass the information back and forth through
a static variable.
An applet can also get references to all other applets on the same page using the getApplets()
method of java.applet.AppletContext. Once you\'ve got a reference to an applet, you can
communicate with it by using its public members.
It is conceivable to have applets in different virtual machines that talk to a server somewhere on
the Internet and store any data that needs to be serialized there. Then, when another applet
needs this data, it could connect to this same server. Implementing this is non-trivial.
Q: What are the steps in the JDBC connection?
A: While making a JDBC connection we go through the following steps :
Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();
Q: How does a try statement determine which catch clause should be used to handle
an exception?
A: When an exception is thrown within the body of a try statement, the catch clauses of the
try statement are examined in the order in which they appear. The first catch clause that is
capable of handling the exceptionis executed. The remaining catch clauses are ignored.
Q: Can an unreachable object become reachable again?
A: An unreachable object may become reachable again. This can happen when the object's
finalize() method is invoked and the object performs an operation which causes it to become
accessible to reachable objects.
Q: What method must be implemented by all threads?
A: All tasks must implement the run() method, whether they are a subclass of Thread or
implement the Runnable interface.
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Q: What are synchronized methods and synchronized statements?
A: Synchronized methods are methods that are used to control access to an object. A
thread only executes a synchronized method after it has acquired the lock for the method's object
or class. Synchronized statements are similar to synchronized methods. A synchronized
statement can only be executed after a thread has acquired the lock for the object or class
referenced in the synchronized statement.
Q: What is Externalizable?
A: Externalizable is an Interface that extends Serializable Interface. And sends data into
Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and
readExternal(ObjectInput in)
Q: What modifiers are allowed for methods in an Interface?
A: Only public and abstract modifiers are allowed for methods in interfaces.
Q: What are some alternatives to inheritance?
A: Delegation is an alternative to inheritance. Delegation means that you include an
instance of another class as an instance variable, and forward messages to the instance. It is
often safer than inheritance because it forces you to think about each message you forward,
because the instance is of a known class, rather than a new class, and because it doesn't force
you to accept all the methods of the super class: you can provide only the methods that really
make sense. On the other hand, it makes you write more code, and it is harder to re-use
(because it is not a subclass).
Q: What does it mean that a method or field is "static"?
A: Static variables and methods are instantiated only once per class. In other words they are
class variables, not instance variables. If you change the value of a static variable in a particular
object, the value of that variable changes for all instances of that class.
Static methods can be referenced with the name of the class rather than the name of a particular
object of the class (though that works too). That's how library methods like System.out.println()
work out is a static field in the java.lang.System class.
Q: What is the difference between preemptive scheduling and time slicing?
A: Under preemptive scheduling, the highest priority task executes until it enters the waiting
or dead states or a higher priority task comes into existence. Under time slicing, a task executes
for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then
determines which task should execute next, based on priority and other factors.
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Q: What is the catch or declare rule for method declarations?
A: If a checked exception may be thrown within the body of a method, the method must
either catch the exception or declare it in its throws clause.
Q: What is the Collections API?
A: The Collections API is a set of classes and interfaces that support operations on
collections of objects.
Q: What is the List interface?
A: The List interface provides support for ordered collections of objects.
Q: What is the Vector class?
A: The Vector class provides the capability to implement a growable array of objects.
Q: What is an Iterator interface?
A: The Iterator interface is used to step through the elements of a Collection .
Q: Which java.util classes and interfaces support event handling?
A: The EventObject class and the EventListener interface support event processing.
Q: What is the GregorianCalendar class?
A: The GregorianCalendar provides support for traditional Western calendars
Q: What is the Locale class?
A: The Locale class is used to tailor program output to the conventions of a particular
geographic, political, or cultural region .
Q: What is the SimpleTimeZone class?
A: The SimpleTimeZone class provides support for a Gregorian calendar .
Q: What is the Map interface?
A: The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with
values.
Q: What is the highest-level event class of the event-delegation model?
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A: The java.util.EventObject class is the highest-level class in the event-delegation class
hierarchy.
Q: What is the Collection interface?
A: The Collection interface provides support for the implementation of a mathematical bag -
an unordered collection of objects that may contain duplicates.
Q: What is the Set interface?
A: The Set interface provides methods for accessing the elements of a finite mathematical
set. Sets do not allow duplicate elements.
Q: What is the typical use of Hashtable?
A: Whenever a program wants to store a key value pair, one can use Hashtable.
Q: I am trying to store an object using a key in a Hashtable. And some other object
already exists in that location, then what will happen? The existing object will be
overwritten? Or the new object will be stored elsewhere?
A: The existing object will be overwritten and thus it will be lost.
Q: What is the difference between the size and capacity of a Vector?
A: The size is the number of elements actually stored in the vector, while capacity is the
maximum number of elements it can store at a given instance of time.
Q: Can a vector contain heterogenous objects?
A: Yes a Vector can contain heterogenous objects. Because a Vector stores everything in
terms of Object.
Q: Can a ArrayList contain heterogenous objects?
A: Yes a ArrayList can contain heterogenous objects. Because a ArrayList stores everything
in terms of Object.
Q: What is an enumeration?
A: An enumeration is an interface containing methods for accessing the underlying data
structure from which the enumeration is obtained. It is a construct which collection classes return
when you request a collection of all the objects stored in the collection. It allows sequential
access to all the elements stored in the collection.
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Q: Considering the basic properties of Vector and ArrayList, where will you use
Vector and where will you use ArrayList?
A: The basic difference between a Vector and an ArrayList is that, vector is synchronized
while ArrayList is not. Thus whenever there is a possibility of multiple threads accessing the same
instance, one should use Vector. While if not multiple threads are going to access the same
instance then use ArrayList. Non synchronized data structure will give better performance than
the synchronized one.
Q: Can a vector contain heterogenous objects?
A: Yes a Vector can contain heterogenous objects. Because a Vector stores everything in
terms of Object.

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